For conventional gas barrier polymers, such as poly(meth)acrylic acid and polyvinyl alcohol which have a highly hydrogen bonding group with high hydrophilicity have been used. Films of these polymers show very high gas barrier properties against oxygen or the like under dry conditions. Under high humidity conditions, however, the gas barrier properties against oxygen or the like are drastically lowered and the films are poor in resistance to moisture or hot water because of the hydrophilicity.
To solve these problems, Patent Document 1 discloses that a polycarboxylic acid polymer layer and a polyvalent metal compound-containing layer are laminated adjacent to each other on a support film, and a polyvalent metal salt of polycarboxylic acid polymer is formed by interlayer reaction. The gas barrier films thus produced are shown to have high oxygen barrier properties even under high humidity.
However, the gas barrier films of Patent Document 1 reduce gas barrier properties or are clouded when exposed to cold water.
Some gas barrier multilayer structures show high gas barrier properties irrespective of humidity or even after retort treated. Patent Document 2 discloses gas barrier multilayer structures that have a layer containing a polycarboxylic acid at least partially neutralized with a divalent or higher polyvalent metal, and a hydrolyzed condensate of a compound which contains a metal atom bonded with at least one characteristic group selected from halogen atoms and alkoxyl groups.
However, the gas barrier multilayer structures disclosed in Patent Document 2 show poor gas barrier properties if they have undergone abuses such as stretching during the production. Handling properties of the multilayer structures are accordingly low.    [Patent Document 1: WO 03/091317]    [Patent Document 2: WO 05/053954]